A fundamental question concerning the cuprates is how and why the metallic state changes with increasing doping from a pseudogap state to a non-Fermi liquid to a Fermi liquid. The authors report Angle Resolved PhotoEmission Spectroscopy [ARPES] measurements on the cuprate LSCO at a doping x=0.23(bulk Tc=25 K) corresponding to overdoping.
From the ARPES data they extract the quasi-particle dispersion and damping rate at different points on the Fermi surface.

Significantly, they find significant variation of the damping rate and its frequency dependence over the Fermi surface. Specifically, a Fermi liquid picture breaks down as one moves away from the zone diagonals, the same region in where the nodes in the superconducting energy gap and the pseudogap appear.
The figure below shows the spectral intensity as one crosses the Fermi surface at different points. The far left is in nodal direction and as one goes to the right one is moving towards the anti-nodal direction. Clearly as one moves from the left to right the quasi-particles are less well defined.

The results are significant for several reasons:

1. they connect the anisotropies of the pseudogap state with the overdoped state.

2. the observed anisotropy goes against theories that assume or claim that the overdoped state is a simple isotropic Fermi liquid.

3. they confirm the anisotropic breakdown of a Fermi liquid proposed previously based on angle-dependent magnetoresistance measurements on overdoped thallium based materials.These ideas were developed more theoretically in this PRL and PRB.

3 comments:

The proposed overdoped phase diagram is essentially what we confirmed experimentally in electron-doped cuprates - see Ref. 3.

We also published a thorough study of quantum criticality on the overdoped side as a function of doping and magnetic field, showing self-consistent transport power laws, critical exponents and B/T scaling! (see our PNAS article).

John,Thanks for your comment.But, I would like to stress that the point of this new paper is not so much the phase diagram [as you deduce from transport] but using ARPES to see the momentum anisotropic character of the non-Fermi liquid.

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I have fun at work trying to use quantum many-body theory to understand electronic properties of complex materials.
I am married to the lovely Robin and have two adult children and a dog, Priya (in the photo). I also write an even more personal blog Soli Deo Gloria [thoughts on theology, science, and culture]

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